This invention relates to a method of manufacturing indium tin oxide (ITO) sputtering targets. In particular, the invention relates to the precipitation and calcining of indium and tin hydroxides, preparation of an aqueous slip from the resultant oxide powders, and compacting the slip in a porous mold using the method of slip casting followed by sintering the resultant compacted target body to yield a high density ITO target.
Transparent conducting thin films of ITO are required for the manufacture of devices such as flat panel display screens and solar cell panels where the ITO film is used to form transparent conducting electrodes. With expansion of the market for flat panel display screens and proliferation of equipment such as Liquid Crystal Display (LCD) screens for televisions, lap top computers and cell phones as well as expansion of the solar cell market, demand for ITO thin films has rocketed. The reason ITO is so commonly used rather than other materials is because of the high light transmission and electrical conductivity of its thin films, and the stability over long service life of equipment, such as LCD televisions, where it is used. The electrically conducting transparent thin films of ITO are known to be produced by a process called sputtering. This is a vacuum deposition method that requires an ITO sputtering target. The latter is a shaped body such as a rectangular tile of high density ITO material.
The quality of the ITO target is very important to obtaining satisfactory sputtering, and for the manufacture of an ITO thin film of quality sufficient for use in LCD related applications, especially LCD's based on Thin Film Transistors (TFT). One key parameter is uniform and high target density across the target body. If the target density is not high and not uniform, then problems are encountered during the ITO sputtering process. The sputtering is carried out using a high vacuum plasma process enhanced by a magnetic field. Normally, a conventional ITO target having a flat rectangular plate shape is used. During sputtering, the material from the target surface is eroded in a shape that mirrors the shape of the local magnetic field. The shape often formed is a curved oval shaped pit called the ‘race track’. This means that the target's utility efficiency is not 100%, and can be as low as 30%. Although the utility efficiency can be improved by recycling the used ITO target, the process becomes unviable if the utility efficiency is further adversely affected by low or non-uniform target density which can cause problems such as ‘nodules’ or target cracking during use. Both of these problems have an adverse effect on ITO thin film quality.
In the prior art, ITO targets were prepared by molding a mixture of indium oxide and tin oxide powders by pressure molding processes such as a hot press molding process to obtain a molded shaped ITO body. However, there are severe limitations to these types of pressure molding processes. One limitation is that modern LCD production lines, called Generation 7, 8 or 10 lines, require large sheets of ITO coated 15 glass. This in turn means that ITO sputtering lines need large size ITO targets. However, when a large ITO target is prepared by a process such as hot pressing, it is difficult to obtain uniform pressure across the target surface, leading to the problems of target warp or cracking. Additionally, such a target suffers from non-uniformity of density as well as non-uniformity of chemical and physical properties across the target body resulting in non-homogenous sputtering across the target surface during the plasma sputtering process. This in turn leads to the formation of nodules on the target surface which reduces the target utilization rate to much less than 30%, since nodules adversely affect ITO thin film quality.
To overcome these issues, prior art discloses the preparation and use of small tiles to prepare large areas by tiling the small tiles together in a 2-dimensional array. In this way, a plurality of small tiles are assembled in an array to give a large surface area target. However, such tiled targets suffer from the problem of catastrophic electric discharges and heat stress chipping at the joints between the small tiles, which not only reduces target utilization rate but also increases the incidence of nodules, and this combination adversely affects the properties of the ITO thin film.
In another method to overcome the above problems, ITO targets are manufactured by using a method known as slip casting which can be carried out under pressure or without pressure.
In one slip casting method (as described, for example, in JP 1117136/88, JP 117137/88, and JP 117138/88), the indium oxide and tin oxide powders are mixed in a liquid such as water with a dispersing agent and a binder to give a slurry called ‘slip’ which is injected into a water absorbing porous mold made of gypsum. The slurry in the mold then slowly dries as the water leaves the mold via the mold pores. This method is called slip casting. The dispersing agent used is, for example, selected from polycarboxylic acids and the binder is selected, for example, from acrylic or polycarbonate emulsions. This process enables ITO targets of a desired shape and size to be obtained simply by changing the shape and size of the mold. In this method, the slurry is injected into a mold under pressure in the range 50 to 200 kPa. Further increases in target density are obtained by subjecting the post dried target to compression by pressure using a cold isostatic press which applies a pressure not less than 100 MPa. The ITO target is then sintered at 1300 to 1400° C. to obtain a dense target with densities greater than 95%. However, this approach suffers from the problem that the yield of targets with densities greater than 99% is low. Further, targets prepared in this way often crack during cold pressing, as well as suffering from nodule formation during sputtering, thereby reducing target utilization below the economically viable threshold. Furthermore, during filling of the mold, the mold material is dispersed into the slurry and adheres to the ITO “green” tile, resulting in contamination of the target by the mold material. These impurities then lead to reduction in the ITO thin film conductivity and problems in the sputtering process such as, for example, the formation of nodules.
In another method (described in JP 2005324987), to prevent such cracking during manufacture, indium and tin oxide powders, water and an organic binder are mixed and then spray dried to yield a granulated powder which is then milled, high pressure press molded and then sintered to yield the ITO target. However, this is a non-slip casting method and suffers from the same problems as described above for non-slip casting methods, such as limitation to small size targets, non uniform densities and abnormal electric discharges during sputtering.
In another embodiment of the prior art (TW 588114B), to overcome the problem of cracking, the ITO target is produced by sintering indium and tin oxides powder mixture, dissolving in aqua regia and filtering with a 0.2 micron filter. After co-precipitation, the oxides are pressed and fired to yield a target which has high density and does not suffer from the problem of nodules. However, this is a long and expensive method. It is also a non-slip casting method so suffers from the problem of size limitation as discussed above.
In another embodiment of the prior art (JP 10330926), to obtain a target that increases utilization during sputtering by minimizing nodules and abnormal electric discharges, the density of the target is regulated to achieve ≧99% and also the maximum diameter of voids existing in the sintered target are regulated to less than or equal to 10 microns with less than 1000 voids in one mm2 area of the target. This is achieved by co-precipitation of indium and tin hydroxides and then calcining in an atmosphere containing hydrogen halide gas such as hydrogen chloride or halogen gas such as chlorine to obtain the corresponding oxides. The oxide powders are then molded into a compact body by slip casting and firing the slip cast green target. In this way, targets of size greater than 100 cm2 can be obtained with densities 2 99%. However, this method is enormously hazardous due to the use of highly poisonous and unstable gases.
In another embodiment of the prior art (JP7243036), to obtain a target that increases utilization during sputtering by minimizing nodules and abnormal electric discharges, an ITO sintered target is produced from a raw material consisting essentially of indium oxide and tin oxide prepared by powder metallurgical engineering. In this case, the average crystal grain diameter is controlled to c4 micron, the number of voids having 3-8 micron average diameter is controlled to <900 void/mm2, the surface roughness Ra is adjusted to <0.5 micron. However, statistically it is very difficult to control all these features simultaneously and ensure consistently good target manufacturing yields.
Hence, there exists a need in the art for systems and methods to obviate or mitigate at least some of the limitations set out above.